A conventional variable depth sonar system includes a boom with a sheave at the outboard end, and frequently another sheave in proximity to the inboard end; an inboard winch with a cable winding drum, a cable running over the sheaves from the cable winding drum to a towed body and means for pivoting the boom to attenuate large variations in cable tension during towing in high Sea States. In a system of this sort, complications and difficulties arise in reeling and unreeling the cable on the drum during hauling in and paying out as a result of lateral deflections of cable between the inboard sheave and drum. Additionally, where the boom is caused to "BOB", that is to pivot to attenuate variations in tension on the cable, the cable runs over the sheaves, producing fatigue in the cable that can result in it parting.
To solve these problems, it has been proposed to arrange the winding drum beside the boom rather than behind it, to use an inboard sheave arranged to lead the cable to the drum along the pivot axis of the boom, and to shift the boom along the drum during paying out and reeling in. This arrangement ensures that the cable is wound without deflection onto the drum. It also ensures that during boom bobbing, there is no cable excursion through the sheaves. However, shifting the boom along the drum during paying out or reeling in of the cable requires a large amount of space onboard the ship and a rather complex system for achieving the necessary movement while ensuring the continued functioning of all active parts of the boom.